基于椭球封闭空气腔的光纤复合法布里-珀罗结构折射率传感特性研究

提出了基于微椭球型空气腔的在线型光纤复合法布里-珀罗干涉结构,并对其折射率传感特性进行了研究.椭球型空气微腔是利用光纤熔接机对实芯光子晶体光纤和单模光纤以特定的熔接参数熔接形成.用高斯光束模型和ABCD法则分析了椭球型空气腔的腔内损耗,建立了电磁场在复合法布里-珀罗干涉结构中传播的物理模型.根据腔长比值的不同,环境折射率对干涉条纹的影响有对比度调制和波长调制,主要研究了一种波长调制型复合法布里-珀罗结构折射率传感器.仿真结果表明该折射率传感器在1—1.6范围内不出现折射率转折点;实验结果表明在1.333—1.466范围内,折射率灵敏度～37.088 nm·RIU-1,分辨率约为2.69×10-5.该光纤复合法布里-珀罗结构干涉条纹对比度高、体积小、成本低,用于折射率测量可靠性高、分辨率高、无折射率拐点、温度串扰小.     

Fractal Aggregation Near the Threshold

In this papel, we present two fractal aggregation models, line pattern seed model (model 1) and point pattern seed model (model 2), which are particle-cluster models. Using the current models, we investigate the critical transition in fractal aggregation processes in two dimensions, and suggest a method for finding the critical transition point. The computer simulation results show that the critical concentration is P_ca=0.69±0.02 for model 1 and P_ca=0.72±0.01 for model 2, critical fractal dimension. D_c= 1.71±0.06 for model 1 and D_c=1.66±0.07 for model 2, which are in good agreement with those of DLA model (D=5/3) and experimental data. The results also show that the critical transition point in two dimensions seems to be inilependent of the size of lattices and the initial seed patterns. The results seem to belong to the same universality class.     

自旋为1的双层平方晶格阻挫模型的相变

采用双时格林函数方法研究了自旋为1的双层平方晶格阻挫模型的相变行为.详细探讨了层间耦合相互作用J_c和单离子各向异性参数D对奈尔态(AF1)和共线态(AF2)之间相转换的影响.结果显示:只要参数J_c和D不同时为零,奈尔态和共线态在J₂=J₁/2(这里J₁和J₂分别描述的是系统自旋间最近邻和次近邻交换作用)时的相变温度相等,两个态共存.在低于相变点的温度范围内,AF1-AF2态之间可以发生相转换,其相变类型为一阶相变.当J₂≠J₁/2时,尽管AF1-AF2态有不同相变温度,但它们也可以共存.如果AF1(AF2)态的相变温度大,在低温,AF1(AF2)态更稳定;在高温,AF2(AF1)态更稳定;在中间温度范围内,AF1-AF2态之间也可以发生一阶相转换.     

Metastability and Spinodal Points for a Random Walker on a Triangle

We investigate time-dependent properties of a single-particle model in which a random walker moves on a triangle and is subjected to nonfocal boundary conditions. This model exhibits spontaneous breaking of a Z ₂ symmetry. The reduced size of the configuration space (compared to related many-particle models that also show spontaneous symmetry breaking) allows us to study the spectrum of the time evolution operator. We break the symmetry explicitly and find a stable phase, and a metastable phase which vanishes at a spinodal point. At this point, the spectrum of the time evolution operator has a gapless and universal band of excitations with a dynamical critical exponent z=1. Surprisingly, the imaginary parts of the eigenvalues E _j(L) are equally spaced, following the rule . Away from the spinodal point, we find two time scales in the spectrum. These results are related to scaling functions for the mean path of the random walker and to first passage times. For the spinodal point, we find universal scaling behavior. A simplified version of the model which can be handled analytically is also presented.     

双层Heisenberg反铁磁体中的量子相变

用键算符平均场方法研究了双层正方格子Heisenberg反铁磁体中的量子相变问题.得到的临界点以及低温下的量子临界行为符合相关的数值计算结果和非线性σ模型的结果.在数值计算不能达到的极低温度范围（T≤0.3J₁,J₁为面内最近邻反铁磁交换作用）,给出了量子临界行为的新证据. 关键词：     

On the projected spectra from a deformed correlated intrinsic state

It is argued that the residual interaction being relatively of long range, should produce Random Phase Approximation (RPA) type of correlations in the Hartree-Fock (HF) intrinsic state. A model is described to take these correlations inti account in the intrinsic state. A comparison of the projected spectra from this state with the exact shell model diagonalization for a model problem bears out this point. An application of the model is distinguishing two almost degenerate HF solutions for the 2s-1d shell nuclei is mentioned.     

Stability and bifurcations of a stationary state for an impact oscillator

The motion of a vibroimpacting one-degree-of-freedom model is analyzed. This model is motivated by the behavior of a shearing granular material, in which a transitional phenomenon is observed as the concentration of the grains decreases. This transition changes the motion of a granular assembly from an orderly shearing between two blocks sandwiching a single layer of grains to a chaotic shear flow of the whole granular mass. The model consists of a mass-spring-dashpot assembly that bounces between two rigid walls. The walls are prescribed to move harmonically in opposite phases. For low wall frequencies or small amplitudes, the motion of the mass is damped out, and it approaches a stationary state with zero velocity and displacement. In this paper, the stability of such a state and the transition into chaos are analyzed. It is shown that the state is always changed into a saddle point after a bifurcation. For some parameter combinations, horseshoe-like structures can be observed in the Poincare sections. Analyzing the stable and unstable manifolds of the saddle point, transversal homoclinic points are found to exist for some of these parameter combinations. (c) 1994 American Institute of Physics.     

Three-sphere low-Reynolds-number swimmer with a cargo container

A recently introduced model for an autonomous swimmer at low Reynolds number that is comprised of three spheres connected by two arms is considered when one of the spheres has a large radius. The Stokes hydrodynamic flow associated with the swimming strokes and net motion of this system can be studied analytically using the Stokes Green's function of a point force in front of a sphere of arbitrary radius R provided by Oseen. The swimming velocity is calculated, and shown to scale as 1/R ³ with the radius of the sphere.     

Kac-Moody-Virasoro symmetry algebra of a （2＋1）-dimensional bilinear system

Based on some known facts of integrable models, this paper proposes a new （2＋1）-dimensional bilinear model equation. By virtue of the formal series symmetry approach, the new model is proved to be integrable because of the existence of the higher order symmetries. The Lie point symmetries of the model constitute an infinite dimensional Kac- Moody Virasoro symmetry algebra. Making use of the infinite Lie point symmetries, the possible symmetry reductions of the model are also studied     

Phase Transitions of a Dilute O(n) Model

We investigate tricritical behavior of the O(n) model in two dimensions by means of transfer-matrix and finite-size scaling methods. For this purpose we consider an O(n) symmetric spin model on the honeycomb lattice with vacancies; the tricritical behavior is associated with the percolation threshold of the vacancies. The vacancies are represented by face variables on the elementary hexagons of the lattice. We apply a mapping of the spin degrees of freedom model on a non-intersecting-loop model, in which the number n of spin components assumes the role of a continuously variable parameter. This loop model serves as a suitable basis for the construction of the transfer matrix. Our results reveal the existence of a tricritical line, parametrized by n, which connects the known universality classes of the tricritical Ising model and the theta point describing the collapse of a polymer. On the other side of the Ising point, the tricritical line extends to the n=2 point describing a tricritical O(2) model.     

Origin of Anisotropy in Gadolinium Crystal Using a New Spin Hamiltonian

Single crystal rare-earth magnets,such as hexagonal-close-packed gadolinium,usually have a large second order anisotropy K_2 and a negative first order anisotropy K_1 at low temperatures,which are difficult to explain using microscopic theories.An atomic scale effective spin Hamiltonian F|{S_i}] is proposed,which,apart from the usual isotropic nearest neighbor coupling J,consists of two new terms that are different for in-plane and out-of-plane neighbors and which are characterized by two new couplings C_1 and C_2,respectively.The hybrid Monte-Carlo method is utilized to sample this system to the desired Boltzmann-like distribution exp(-F/k_BT).It is found that K_2 and K_1 are compatible with the experimental values and arise naturally from the exchange anisotropy C_1 and C_2,which are less than 0.01% in magnitude of the isotropic exchange energy J.This new model spin Hamiltonian can also be applied to study other magnetic properties.     

Universal low-temperature properties of frustrated classical spin chain near the ferromagnet-helimagnet transition point

The thermodynamics of the classical frustrated spin chain near the transition point between the ferromagnetic and the helical phases is studied. The calculation of the partition and spin correlation functions at low temperature limit is reduced to the quantum mechanical problem of a particle in potential well. It is shown that the thermodynamic quantities are universal functions of the temperature normalized by the chiral domain wall energy. The obtained behavior of the static structure factor indicates that the short-range helical-type correlations existing at low temperatures on the helical side of the transition point disappear at some critical temperature, defining the Lifshitz point. It is also shown that the low-temperature susceptibility in the helical phase near the transition point has a maximum at some temperature. Such behavior is in agreement with that observed in several materials described by the quantum s = 1/2 version of this model.     

Rigorous solution of the spin-1 quantum Ising model with single-ion anisotropy

We solve the spin-1 quantum Ising model with single-ion anisotropy by mapping it onto a series of segmented spin-1/2 transverse Ising chains, separated by the S(z)=0 states called holes. A recursion formula is derived for the partition function to simplify the summation of hole configurations. This allows the thermodynamic quantities of this model to be rigorously determined in the thermodynamic limit. The low temperature behavior is governed by the interplay between the hole excitations and the fermionic excitations within each spin-1/2 Ising segment. The quantum critical fluctuations around the Ising critical point of the transverse Ising model are strongly suppressed by the hole excitations.     

Extended hubbard model with ring exchange: a route to a non-Abelian topological phase

We propose an extended Hubbard model on a 2D kagome lattice with an additional ring exchange term. The particles can be either bosons or spinless fermions. We analyze the model at the special filling fraction 1/6, where it is closely related to the quantum dimer model. We show how to arrive at an exactly soluble point whose ground state is the "d-isotopy" transition point into a stable phase with a certain type of non-Abelian topological order. Near the "special" values, d=2cos(pi/(k+2), this topological phase has anyonic excitations closely related to SU(2) Chern-Simons theory at level k.     

Crossed spin-1/2 heisenberg chains as a quantum impurity problem

Using equivalencies between different models we reduce the model of two spin-1/2 Heisenberg chains crossed at one point to the model of free fermions. The spin-spin correlation function is calculated by summing the perturbation series in the interchain interaction. The result reveals a power law decay with a nonuniversal exponent.     

Critical properties of the N-color london model

The critical properties of the N-color London model are studied in d=2+1 dimensions. The model is dualized to a theory of N vortex fields interacting through a Coulomb and a screened potential. The model with N=2 shows two anomalies in the specific heat. From the critical exponents alpha and nu, the mass of the gauge field, and the vortex correlation functions, we conclude that one anomaly corresponds to an inverted 3Dxy fixed point, while the other corresponds to a 3Dxy fixed point. There are N fixed points, namely, one corresponding to an inverted 3Dxy fixed point, and N-1 corresponding to neutral 3Dxy fixed points. This represents a novel type of quantum fluid, where superfluid modes arise out of charged condensates.     

Thermodynamics of a model for valence fluctuations between two magnetic configurations

The thermodynamic Bethe-Ansatz equations of a model describing valence fluctuations between two magnetic configurations are derived. The model includes a spin-1/2 and a spin-1 configurations hybridized through the promotion of an electron to a conduction band.Thes–d like limits are reproduced. An expression for the specific heat at low temperatures is obtained.ConicetGuggenheim fellow     

Modulated spin liquid: a new paradigm for URu2Si2

We argue that near a Kondo breakdown critical point, a spin liquid with spatial modulations can form. Unlike its uniform counterpart, we find that this occurs via a second order phase transition. The amount of entropy quenched when ordering is of the same magnitude as for an antiferromagnet. Moreover, the two states are competitive, and at low temperatures are separated by a first order phase transition. The modulated spin liquid we find breaks Z4 symmetry, as recently seen in the hidden order phase of URu2Si2. Based on this, we suggest that the modulated spin liquid is a viable candidate for this unique phase of matter.     

Junction of three off-critical quantum Ising chains and two-channel Kondo effect in a superconductor

We show that a junction of three off-critical quantum Ising chains can be regarded as a quantum spin chain realization of the two-channel spin-1/2 overscreened Kondo effect with two superconducting leads. We prove that, as long as the Kondo temperature is larger than the superconducting gap, the equivalent Kondo model flows towards the two channel Kondo fixed point. We argue that our system provides the first controlled realization of two channel Kondo effect with superconducting leads. Besides its theoretical interest, this result is of importance for potential applications to a number of contexts, including the analysis of the quantum entanglement properties of a Kondo system.     

On a sharp transition from area law to perimeter law in a system of random surfaces

We introduce and study a phase transition which is associated with the spontaneous formation of infinite surface sheets in a Bernoulli system of random plaquettes. The transition is manifested by a change in the asymptotic behavior of the probability of the formation of a surface, spanning a prescribed loop. As such, this transition offers a generalization of the bond percolation phenomenon. At low plaquette densities, the probability for large loops is shown to decay exponentially with the loops' area, whereas for high densities the decay is by a perimeter law. Furthermore, we show that the two phases of the three dimensional plaquette system are in a precise correspondence with the two phases of the dual system of random bonds. Thus, if a natural conjecture about the phase structure of the bond percolation model is true, then there is a sharp transition in the asymptotic behavior of the surface events. Our analysis incorporates block variables, in terms of which a non-critical system is transformed into one which is close to a trivial, high or low density, fixed point. Stochastic geometric effects like those discussed here play an important role in lattice gauge theories.